The invention relates to a steering angle adjusting device for a motor vehicle, said device comprising an operating element by which the steering angle can be adjusted as a function of a corresponding operating element actuating parameter such as the deflection of the operating element or the force exerted on said element.
Conventionally, a steering wheel serves as the operating element for adjusting the steering angle in automobiles, with the steering wheel actuating force being transmitted directly by mechanical means to the vehicle wheels. At the same time, the user experiences a feedback at the steering wheel that is a function of the situation and indicates to him how easily the steering angle can be changed in a given situation. Steering systems are already known in which the steering wheel is eliminated, such as when a joystick is used as the operating element to adjust the steering angle. In such systems, the adjustment of the steering angle takes place as a function of the deflection of the joystick or the actuating force exerted on the stick as a corresponding operating element actuating parameter, for which a corresponding characteristic is provided. The actuation of the operating element is decoupled mechanically from the steering angle movement of the vehicle wheels and the latter is performed instead by an adjusting drive that is controlled by a steering angle control unit as a function of the detected operating element actuating parameter.
A steering angle adjusting device of the latter type is known from the article by P. Branneby et al., "Improved Active and Passive Safety by Using Active Lateral Dynamic Control and an Unconventional Steering Unit," 13th International Technical Conference on Experimental Safety Vehicles, Proceedings, Vol. 1, Nov. 4-7, 1991, Page 224, the disclosure of which is incorporated by reference herein as non-essential material. In this device a nonlinear characteristic of the steering angle is set as a function of the operating element deflection in such fashion that larger steering angle changes are performed with greater sensitivity than for smaller steering angle changes. In other words, a given change in deflection of the operating element results in a smaller steering angle change in the range of small steering angles than in the range of large steering angles.
In another steering angle adjusting device of this type, as described in H. Bubb, "Arbeitsplatz Fahrer-Eine ergonomische Studie [The Driver Workstation: an Ergonomic Study]," Automobil-Industrie, 3/85, Page 265, an operating element located approximately in the position of the steering wheel serves firstly to control the lengthwise dynamics by displacement parallel to the lengthwise direction of the vehicle and secondly to adjust the steering angle by twisting.
U.S. Pat. No. 3,022,850 describes a steering angle setting device in which a joystick serves simultaneously for adjusting the steering angle and for controlling the lengthwise dynamics of the vehicle, the specification of which is expressly incorporated by reference herein. In connection with the steering functionality, it is proposed that the joystick deflection signal be amplified as a function of speed, preferably in inverse proportion to the square of the lengthwise velocity so that a given joystick deflection leads to an essentially constant transverse acceleration that is independent of the lengthwise velocity of the vehicle.
The technical problem to be solved by the invention is to provide a steering angle adjusting device of the above-mentioned type that permits safe steering of the vehicle in terms of driving dynamics, especially for a system in which the angular movement of the vehicle wheels is not coupled mechanically with operating element actuation to adjust the steering angle.
This problem is solved by a steering angle adjusting device for a motor vehicle with an operating element for adjusting the steering angle as a function of a corresponding operating element actuating parameter. Steering angle adjustment is performed as a function of the operating element actuating value (x) with a sensitivity that decreases as the coefficient of friction (.mu.) decreases. Further, it is contemplated that with an operating element for adjusting the steering angle as a function of a corresponding operating element actuating value, the maximum adjustable value (x.sub.t) of the operating element-actuating value (x) in each case decreases with decreasing coefficient of friction (.mu.) and/or increasing vehicle lengthwise and/or transverse motion (v).
According to the invention, the steering angle adjustment is performed with a sensitivity that depends on the current coefficient of friction in such fashion that steering angle adjustment sensitivity is reduced as the road surface becomes smoother and the coefficient of friction decreases. In this way, when the vehicle reaches a smoother road surface, the risk of excessive steering interventions on the wheels, which could result in unstable driving behavior, is prevented. A similar effect is achieved by the measure for reducing the value of the operating element actuating parameter, which is the maximum that can be set in the given situation, with a declining coefficient of friction and increasing vehicle lengthwise velocity, so that the maximum steering angle that can be set is correspondingly reduced in a simple fashion. This measure can possibly be provided in addition to the above-described measure.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.